1. Field of the Invention
The present invention relates to data compression and, more particularly, to data compression provided as a function on a digital still video camera.
2. Description of the Related Art
Conventional digital still video cameras conduct photographing of still pictures using a solid state imaging device, such as a CCD sensor, and store the obtained photographic image data on a semiconductor memory card. In order to provide effective utilization of the memory card, the image data is usually compressed.
FIG. 5 is a block diagram which illustrates a configuration of a conventional digital still video camera. In FIG. 5, a photographic lens 501, which is generally a group of lenses, conducts image formation of the subject image. Light from the subject passes through an aperture 502 and a mirror optical system 503 designed to introduce the subject light into a photometric part 516, such as a photometer, and range finding device (not illustrated). Photometric part 516 and the range finding device provide exposure control and focus control, respectively. After passing through mirror optical system 503, the light from the subject passes through a shutter 504. An imaging part 505 converts the subject light into electric signals and outputs image signals. Imaging part 505 includes a solid state imaging device (not illustrated) such as a CCD sensor.
A/D converter 506 receives the image signals from imaging part 505 and converts the image signals into digital signals. A signal processor 507 receives the digital signals from A/D converter 506 and conducts constant signal processing. Such signal processing can include white balance correction, amplification, g correction, and profile highlighting. A buffer memory 508 temporarily stores the image signals which have been processed by signal processor 507. A compression circuit 509 compresses the image data and memory card 510 stores the compressed image data.
Control part 520, generally a microprocessor, controls all photographic operations. Lens 501, aperture 502, mirror optical system 503, shutter 504, imaging part 505, A/D converter 506, signal processor 507, buffer memory 508, compression circuit 509, memory card 510 and photometric part 515 are electrically connected to control part 520. A focus detector 515 is also electrically connected to control part 520. Computation for focusing and optimum exposure is conducted by control part 520 based on signals received from focus detector 515 and photometric part 516. The driving of lens 501, aperture 502 and shutter 504 is controlled based on the computation results. In this manner, automatic focusing and automatic exposure are conducted.
The degree of compression (that is, the compressibility) of the image data in compression circuit 509 is preselected by a manual switch (not illustrated). The photographer selects the desired compressibility by using this manual switch.
A conventional camera as described in FIG. 5 is complex to operate since it requires the manipulation of a manual switch to select the compressibility of the data. Moreover, this selection must be performed prior to photographing. This manual operation is contrary to current trends in camera operation to reduce camera operating procedures and simplify camera use for the photographer.
In order to eliminate the use of a manual switch, there are camera designs in which the digitized image data stored in buffer memory 508 is analyzed and the compressibility automatically determined. Other designs optimize compressibility by partially extracting image data from buffer memory 508, compressing the extracted image data, conducting feedback of the results, and repeating the compression.
However, in either of these cases, rapid photographing cannot be conducted because a relatively long time is required to calculate the optimum compressibility. In addition, a relatively large amount of electric power is consumed since the electric power consumed is proportional to the operation of the compression circuit.